mtd: nand_base: use __func__ instead of typing names

Correcting debug prints by removing function names from print messages
and using '__func__' macro instead.

Function names were wrong in few places.

Signed-off-by: Vimal Singh <vimalsingh@ti.com>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>